Descend visually
Have definately used this phrase, almost on a daily basis. If an a/c requests a visual app but you can't release him own nav at that time, then 'continue heading ... descend visually' or words to that effect are used by most at my unit. Then when whatever was restricting the app is sorted 'C/S cleared vis app rwy ...no descent restriction' The no restriction bit may not be necessary but confirms the new clearance and prevents the inevitable 'can we descend too ?'